Pressurized powder support for treating processes

ABSTRACT

A method for supporting a piece which is to undergo a treatment process including embedding the piece in a particulate support medium, enclosing the support medium in a container, and pressurizing the support medium by applying pressure to the outside of the container or by applying a vacuum to the interior of the container. Various treatment processes may be employed, such as annealing.

This is a continuation of Ser. No. 07/451,416, filed on Dec. 15, 1989,now abandoned.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

This invention relates to supporting a fabricated piece for purposes oftreating said fabricated piece and in particular to a method forsupporting a thermoplastic piece for annealing.

2. Background Of The Art

Methods for annealing plastics are commonly known in the art. Ingeneral, plastics are annealed to remove residual stresses and strains,to stabilize the shape of a plastic piece, and to reduce or eliminatedefects in the piece. After forming a plastic piece by means ofinjection molding or extrusion, residual flow induced stresses mayremain in the piece, as well as stresses from the differential density,shrinkage, and the viscoelastic behavior of the plastic duringnonuniform cooling. Stresses are usually induced by quickly cooling apolymer from above its glass transition temperature. Moldedthermoplastics containing residual stresses are more likely to developstress crazing and stress cracking. To alleviate the effects of internalstresses, plastic pieces are usually annealed. Annealing typicallycomprises reheating the plastic to above its glass transitiontemperature where chain mobility is greatest, and then slowly andgradually cooling the plastic to avoid reintroducing stresses. Plasticwhich is heated above its glass transition temperature usually relievesinternal stresses by changes in density and specific volume.

Moreover, some degree of crystallization of the plastic may be achievedby heating it to a temperature above its glass transition temperature.Consequently, a freely held piece will often undergo distortion andshrinkage. Presently used equipment for annealing plastic usuallycomprises either liquid baths or hot air. Liquid bath methods usingwater, oil or waxes, usually make some provision for recycling theliquid to prevent temperature build-up or hot spots. Air flow methodsusually employ circulating-air or air flow ovens. Other methods such asusing convection ovens infrared ovens, and dielectric (e.g. microwave)ovens are also employed. Generally such methods require means such asracks or clamps to hold or suspend the plastic piece in the annealingoven.

Plastic pieces used in surgical procedures often have complicatedstructures and must operate in conditions where close tolerances arerequired. Even small distortion or shrinkage in plastic pieces such assurgical fasteners is undesirable. Racks or simple clamping means arenot effective for eliminating distortion or shrinkage, hence the use offixturing, which involves supporting the plastic piece alongsubstantially its entire external surface, usually by placing theplastic piece in a device closely resembling the mold from which thepiece was originally fabricated. Pieces which have fine surface detailrequire expensive fixtures to match and support the detailedconfiguration of the piece.

Annealing is but one example of various processes which require a workpiece to be supported in this manner. Other treating processes whichrequire access to the surface of the work piece, such as those entailingchemical reactions, diffusion, coating, etc., often cannot beefficiently performed with the customary fixturing apparatus andmethods.

To overcome problems such as those described above, the presentinvention provides a novel and inexpensive method to support a workpiece for undergoing treatment processes, such method providing supportover substantially the entire surface of the work piece while allowingaccess to the surface for various treatment and conditioning processes.The invention further provides a unique method for supporting a workpiece having at least one hollow or cut-out section either internally orexternally so as to maintain an equilibrium between internal andexternal pressure forces applied to the work piece. The invention alsoprovides a surgical fastener constructed of a material treated inaccordance with the method disclosed herein.

SUMMARY OF THE INVENTION

A method is provided herein for supporting an object to be treatedcomprising placing the object within a particulate support medium, andcompacting the support medium into a rigid mass by applying pressurizingmeans to create a difference between the pressure outside theparticulate support medium and the pressure inside the particulatesupport medium. The pressurizing means can comprise reducing thepressure inside the support medium as, for example, by applying a vacuumto the interior of the support medium, preferably with an insidepressure of less than 1 torr. The support medium is optimally containedwithin a flexible container such as a flexible bag if application of avacuum is the pressurizing means. The pressurizing means also cancomprise increasing the pressure outside the particulate support mediumsuch as by applying pressure to the particulate support medium by apiston means wherein the piston means is hydraulically driven,pneumatically driven, spring driven, or driven by deadweight. Theparticulate support medium is ideally contained in a rigid container ifa piston means is employed. The object can then be treated by, forexample, annealing the object. The particulate support medium cancomprise powdered talc, or sodium bicarbonate, preferably sodiumbicarbonate which has been conditioned by treating for about 12 hours atabout 125° C., or any other suitable particulate material.

An alternative embodiment provides a method for supporting an object fora treating process comprising placing the object within a particulatesupport material, enclosing the object and particulate support materialwithin a rigid container which is sealed on all sides, the particulatesupport material and the container having different coefficients ofthermal expansion, and compacting the particulate support medium bychanging the temperature of the support medium and the container.Wherein the particulate support material has a coefficient of thermalexpansion greater than that of the container, changing the temperatureis accomplished by heating the container and particulate support medium.Wherein the particulate support medium has a coefficient of thermalexpansion less than that of the container, changing the temperaturecomprises cooling the container and particulate support medium.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention are describedhereinbelow, wherein:

FIG. 1 is a diagrammatic elevational view partially in cross section ofan apparatus employing the vacuum method of the present invention.

FIG. 2a and 2b are plan and elevational views, respectively, of aretainer portion of a two part surgical fastener in a powder support.

FIG. 3 is a diagrammatic elevational view of an apparatus employing theexternal pressurization method of the present invention.

FIGS. 4a and 4b are elevational and bottom views, respectively, of asurgical fastener annealed in accordance with the method of the presentinvention.

FIG. 5 is an elevational cut away view, partially in cross section of anapparatus employing the thermal method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The method of the present invention utilizes a pressurizing means tocompress a body of particulate support medium in which a piece to betreated is embedded.

The work piece contemplated is any object on which it is desired toeffect a treatment process. A preferred work piece is constructed of anannealable material such as molded thermoplastics, or the like. Inparticular, the application of the method of the present invention tobioabsorbable surgical fastener is contemplated.

Although the pressurized powder support of the present invention isparticularly suitable for annealing, other methods of treatment whichentail coating a work piece, diffusion of materials into or out of thework piece, chemical reactions with the work piece, etc. are alsocontemplated as being within the scope of the present invention.

"Pressurizing" is defined as establishing a pressure difference suchthat the pressure external to the body of particulate medium is greaterthan the pressure internal to the body of particulate medium, therebyresulting in compression of the medium. Such pressurizing may beachieved by increasing the external pressure or by decreasing theinternal pressure.

Increasing the external pressure may be accomplished, for example, by apiston actuated by hydraulic or pneumatic means, by spring mechanism, orby a dead weight applied to the piston. Another way to increase theexternal pressure is by a pre-stressed membrane, i.e. for exampleplacing the particulate medium in a pre-stretched elastic membrane andthen allowing the membrane to contract.

Decreasing the internal pressure may be accomplished by applying avacuum to the interior of the particulate medium. A preferred method forthe use of an evacuated powder support medium for treatment processes isdescribed below.

Referring to FIG. 1 the thermoplastic work piece 12 to be annealed isembedded in a finely divided support medium 14 contained within a pan orreceptacle 16. Work piece 12 may be of any shape or configuration and isgenerally produced from an injection molding process. The support medium14 may be any inert powder of suitable particle size to rigidly supportthe piece. Because work piece 12 may be only a few millimeters in size,and of relatively complex shape, it is important that the particle sizeof powder 14 be sufficiently small relative to the work piece so as toenable the powder to conform to and to support the intricate surfacedetail of piece 12. Powdered talc has been found to be useful. However,it is preferable to employ a powder which is water soluble and can bewashed free from the piece 12 after annealing. For example, sodiumbicarbonate has been found to be an effective support medium. The sodiumbicarbonate powder is preferably conditioned by heating in an oven inflat pans at about 125° C. for about 12 hours, whereby it is largelyconverted to the carbonate. The powder is then cooled and sifted through#20 standard mesh screen and stored dry.

The particles of the support medium 14 are able to flow around workpiece 12 and to conform to the details of the exterior surface of thepiece. A fine mesh screen 19 is positioned above the layer of supportmedium 14 to prevent it from being drawn out by the vacuum. The pan 16containing the support medium 14 and embedded piece 12 is placed into aflexible air tight vacuum bag 18. Vacuum bag 18 may completely enveloppan 16 although this is not essential, provided that a substantiallyairtight enclosure of support medium 14 is accomplished. The apparatusis placed in oven 20 and connected to a vacuum line 24 by means of valve22, and a vacuum is applied to the interior of bag 18 to reduce theinterior pressure preferably to about 1 torr.

The application of a vacuum to bag 18 serves to create a pressuredifferential between the interior and exterior of bag 18. Thus, forexample, if the external pressure is a standard atmosphere, i.e. 760torr, reducing the pressure within bag 18 through the application of avacuum to 1 torr will create a pressure differential of almost oneatmosphere. The maximum pressure differential obtainable by vacuum meansalone is atmospheric pressure, or about one atmosphere. However, higherpressure differentials may be achieved by increasing the externalatmospheric pressure, e.g. by placing bag 18 in a pressure oven. Thus,alternative methods (i.e. increasing the pressure of the surroundingatmosphere, and decreasing the internal pressure) may be used singly orin conjunction with each other to achieve a broad range of pressuredifferences. The ability to adjust the pressure differential is a highlydesirable feature since the work pieces to be treated or the treatmentprocesses to be employed may have characteristics which require certainpressure differences or certain internal pressures.

Preferably, a vacuum bag sealer (not shown) may be used to maintain thevacuum within bag 18. After drawing the vacuum, bag 18 is sealed as aunit. Disposable plastic bags may be employed. Use of a vacuum bagsealer eliminates the need for the bag to be constantly attached to avacuum source. When the interstitial air is evacuated the particles ofthe support medium 14 are compacted into a rigid mass for as long as thevacuum is maintained. Thus, the powder 14 which was once fluid istemporarily rigidified into an inflexible mold-like support whichpresents an inner surface conforming in exact detail to the shape ofwork piece 12.

A preferred application of the method of the present invention isdirected to annealing surgical fasteners such as fastener 40 illustratedin FIGS. 4a and 4b, and retainer portion 50 of a two part surgicalfastener as illustrated in FIGS. 2a and 2b. Such fasteners are oftenmolded from thermoplastic materials which are bioabsorbable (i.e.absorbed by the human or animal body in which they are implanted).Because of the particularly sensitive environment and use contemplatedby such fasteners, it is of utmost importance to anneal them toeliminate potential defects and stresses which would present unnecessarydanger to patients. As noted hereinabove, however, although suchsurgical fasteners are contemplated as a preferred application of theinstant method, it is clearly applicable to all products constructed ofthermoplastic materials where annealing is necessary or desirable.

One of the advantages of the method of the present invention is that thepowder support medium can fill the hollow spaces of the interior of apiece so that when the powder support medium is pressurized, the powdermedium in the interior to the work piece is pressurized as much as thepowder exterior to the work piece, thereby establishing a pressureequilibrium. FIG. 2a illustrates in plan view a retainer portion 50 of atwo part surgical fastener, which is embedded in the powder supportmedium 14. FIG. 2b illustrates the retainer 50 in elevational partiallysectional view. As can be seen the powder medium 14 enters and fills thehollow channel 52 extending through columnar members 51. Because thepowder medium is often poured or sprinkled from above the container 16,it is often advantageous to facilitate entry of the powder 14 into ahollow space by orienting the work piece such that the powder naturallyfalls into the hollow channel. Thus, retainer 50 is preferably orientedso that the opening 52a of hollow interior channel 52 is aligned withthe inflow of powder medium 14, i.e. by being placed vertically upright.Powder 14 fills the channel 52 as it is being poured into container 16and exerts pressure on the interior walls of the column to balanceexternal pressure.

Referring again to FIG. 1, if it is desired to anneal a work piece 12,oven 20 may be heated and held at a temperature and time sufficient toanneal the polymer. The appropriate temperature and heating time willdepend upon the type of thermoplastic resin from which the plastic pieceis constructed, the size and shape of the piece, the heatingcharacteristics of the apparatus and support medium, etc. Convectionheating is generally used, but other types of heating such as dielectricheating, microwave heating, etc. may also be employed.

When the heating has been completed, the apparatus is allowed to coolslowly to ambient or room temperature. The contents of the pan 16 areremoved and separated by, for example, sifting on a standard mesh screento separate the plastic work piece 12 from the powder 14. If a watersoluble powder is used, such as sodium bicarbonate, piece 12 can bequickly rinsed in water to remove any traces of powder, then vacuumdried to remove the water.

FIG. 3 is a diagrammatic view of alternative ways to pressurize thesupport medium. The thermoplastic work pieces such as the aforementionedsurgical fastener 40, and retainer 50, which are to be treated, areplaced in particulate support medium 14 contained within a rigidcontainer 16. As noted, support medium 14 is preferably fine powder,such as conditioned sodium bicarbonate or talc. Piston 28 pressesdownwardly upon the support medium 14 by means of an applied force F,which may be applied by hydraulic or pneumatic means, or by a springmeans, or by dead weight.

The preferred sequence of operation is as follows. A quantity of powdersupport medium such as conditioned sodium bicarbonate or talc is placedin a cylindrical container and tamped down with piston 28 to produce aflat reference surface. Thermoplastic pieces such as surgical fastenersand retainers are placed on this surface and additional powder isapplied and tamped. Alternate layers of support medium and plasticpieces are gradually built-up. Finally the piston is pressed down on theupper surface of the last layer of medium and secured by clamping means.The treatment process such as annealing is then performed as describedhereinabove.

EXAMPLE 1

Sodium bicarbonate (baking soda) was conditioned in an oven in flat pansat 125° C. for 12 hours. After cooling, the powder was sifted through a#20 standard mesh screen and stored dry. An aluminum pan (7 in.×11 in.×1in.) was partially filled with a layer of the conditioned sodiumbicarbonate. Parts to be annealed were then pressed by hand into theconditioned sodium bicarbonate powder. Additional conditioned sodiumbicarbonate was added to fill the pan and to fill the hollow and cut-outsections of the parts. A thin, finely porous material, namely a paperhairnet, was used to cover the pan to hold the powder in place whileapplying a vacuum.

The pan was placed in a Fairprene rubberized vacuum bag (11 in.×16 in.inside dimensions) equipped with a vacuum disconnect fitting. The openend of the bag was clamped tightly closed. The bag was then placed in aconvection oven equipped with a vacuum line. The vacuum bag wasconnected to the vacuum line and evacuated (less than 1 Torr) andmaintained under vacuum. The oven was heated to 100° C. for three hours,which was a sufficient temperature and time to crystallize the polymer.

When the heating cycle was completed, the vacuum bag and its contentswere allowed to cool to room temperature before removing the pan. Thecontents of the pan were sifted on a standard 18 mesh screen to separatethe parts from the powder. The parts were quickly rinsed in water toremove any traces of powder and then placed in a vacuum oven at lessthan 1 Torr to remove residual water.

EXAMPLE 2

In this example the shrinkage associated with the prior known fixturingmethod of annealing is compared with the shrinkage associated with thepressurized powder method of the present invention.

The pieces annealed by both methods were surgical fasteners constructedfrom 90% glycolide/10% lactide copolymer and are illustrated in FIGS. 4aand 4b. Each fastener 40 comprised a backspan 41 and two prongs 42projecting from the backspan 41. The prongs 42 each had barbs 43 attheir tips.

Three 12-piece lots of fasteners were provided, each lot of fastenersoriginating from a different lot of plastic construction material. Thepieces were first measured in the unannealed state for six dimensions:backspan length, backspan width, backspan thickness, the distancebetween prongs, the distance between the end of the backspan and theprong, and the diameter of the prong.

Each lot was divided such that 6 pieces were fixture annealed inaccordance with the prior known fixturing method and 6 pieces werepowder annealed in accordance with the method of the present invention.After annealing each fastener was again measured with respect to thedimensions indicated above. The average percentage of shrinkage wasdetermined in accordance with the following formula ##EQU1## wherein X₁=the average of the measured dimensions prior to annealing

X₂ =the average of the measured dimensions after annealing

The results are tabulated in Table 1.

                  TABLE 1                                                         ______________________________________                                        Average Percentage of Shrinkage                                               Lot  Fixtured Pieces                                                                            Pressurized Powder Supported Pieces                         ______________________________________                                        1    6.7%         2.4%                                                        2    5.8%         3.2%                                                        3    2.8%         1.5%                                                        ______________________________________                                    

The above results clearly indicate that the pressurized powder annealingmethod of the present invention is associated with significantly lessshrinkage than the prior known fixturing method. Thus greaterdimensional stability of finished end use products is clearly possible.

In yet another embodiment of the present invention, as exemplified byFIG. 5, the pressure differential is not established by having a higherpressure external to the medium. Rather, a pressure differential isestablished between the powder medium and the object to be treated.Unlike the previously described embodiments in which the container wasin some way deformable in response to a pressure differential betweenthe interior and exterior (as, for example, by flexibility of thecontainer or, if rigid, by means of a movable piston) so as to compressthe support medium, the embodiment exemplified in FIG. 5 utilizes adifference in the coefficient of thermal expansion between the materialwhich the particulate support comprises, and the material from which thecontainer is made. A difference in thermal expansion will causecompression of the particulate support medium when the apparatus issubjected to a temperature change.

In FIG. 5, a powder medium 14 is tightly packed around a thermoplasticpiece 12, and is contained in rigid container 16 which is sealed on allsides. Lid 30 and container 16, for example, may be coupled together bymeans of a threaded joint. The powder medium 14 occupies substantiallyall of the free space of the interior of the container. There isessentially no gap between the upper surface level of the powder medium14 and the bottom surface 30a of the lid.

If the coefficient of thermal expansion of the support medium is greaterthan that of the container, the particulate medium will be compressed byplacing the apparatus in a heating environment: the expanding particlesare confined within a container which expands to a lesser degree.

Conversely, if the coefficient of thermal expansion of the container isgreater than that of the particulate support medium, the particulatesupport medium will be compressed if the apparatus is cooled, thecontainer contracting to a greater degree than that of the medium. Thechoice of materials, therefore, must be chosen to conform to thetemperature requirements of the treatment process to be undergone. Ifthe piece is to be subjected to an annealing process, and, therefore,heated to the appropriate temperature, the thermal expansion of theparticulate medium must be greater than that of the container. Heatingthe apparatus will then cause the particulate medium to expand within aconfined space, thereby creating a compression force on all sides of thepiece while the piece is being treated.

The pressurized powder support of the present invention may be used forprocesses other than annealing plastic. For example, the particles canbe employed as a carrier for a coating material. Various agents may bemixed with the powder medium for diffusion into the surface of thesupported piece. The pressurized powder medium may be used to supportpieces to be case hardened.

While the above description contains many specifics, these specificsshould not be construed as limitations on the scope of the invention,but merely as exemplifications of preferred embodiments thereof. Thoseskilled in the art will envision many other possible variations that arewithin the scope and spirit of the invention as defined by the claimsappended hereto.

What is claimed is:
 1. A method for annealing a surgical objectcomprising:a) providing a container; b) placing a particulate supportmedium with the container, said particulate support medium comprising anaggregation of substantially rigid particles of sufficiently small sizeto substantially conform to an outer surface of the object; c) placingan already formed surgical object in said particulate support medium inthe container, said support medium thereby providing at least a bottomsupport for said surgical object, said surgical object being composed ofa biocompatible thermoplastic material; d) placing additionalparticulate support medium within the container; and thereafter e)compacting the particulate support medium into a substantially rigidmass to substantially conform to the outer surface of said surgicalobject, said particulate support medium substantially enveloping saidsurgical object and supporting the surgical object within the container;and then f) applying heat for annealing the surgical object.
 2. A methodas recited in claim 1, wherein said step of compacting the supportmedium into a substantially rigid mass comprises the step of creating adifference between the pressure exterior to the aggregation of particlesof the particulate support medium and the pressure inside theparticulate support medium.
 3. A method as recited in claim 1, whereinsaid container is substantially rigid on all sides and said particulatesupport medium and said container have relatively different coefficientsof thermal expansion and the step of compacting said particulate supportmedium comprises changing the temperature of said particulate supportmedium and said container.
 4. A method as recited in claim 1, whereinsaid surgical object is fully covered by said particulate supportmedium.
 5. A method as recited in claim 1, wherein the step of placingthe surgical object within the container further comprises the step ofpressing the surgical object into the particulate support medium.
 6. Themethod of claim 2, wherein the step of creating a difference between thepressure exterior to the aggregation of particles of the particulatesupport medium and the pressure inside the particulate support mediumcomprises applying a vacuum to the interior of the support medium. 7.The method of claim 1, wherein said particulate support medium comprisespowdered talc.
 8. The method of claim 1, wherein said particulatesupport medium comprises sodium bicarbonate.
 9. The method of claim 1,wherein said particulate support medium remains porous after beingcompacted into a substantially rigid mass.
 10. The method of claim 1,wherein said particulate support medium is water soluble.
 11. A methodfor annealing a predetermined plurality of surgical objectscomprising:(a) providing a container; (b) placing a layer of particulatesupport material within said container, said particulate supportmaterial comprising an aggregation of substantially rigid particles ofsufficiently small size to substantially conform to an exterior surfaceof each of the surgical objects; (c) placing a plurality of alreadyformed surgical objects within the container on said first layer of saidsupport material, said first layer of support material providing atleast a bottom support for said surgical objects, said surgical objectsbeing composed of a bio-compatible thermoplastic material; (d) placing asecond layer of particulate support material within the container tosubstantially cover said surgical objects; and thereafter, (e)compacting the support material into a substantially rigid mass tosubstantially conform to the outer surface of the objects and to supportthe surgical objects within the container; and then (f) applying heatfor annealing the surgical objects.
 12. A method as recited in claim 11,wherein said step of compacting the support medium into a substantiallyrigid mass comprises the step of creating a difference between thepressure exterior to the aggregation of particles of the particulatesupport material and the pressure inside the particulate supportmaterial.
 13. A method as recited in claim 11, wherein said container issubstantially rigid on all sides and said particulate support medium andsaid container have relatively different coefficients of thermalexpansion and the stp of compacting said particulate support mediumcomprises changing the temperature of said particulate support mediumand said container.
 14. A method for annealing a surgical objectcomprising:(a) placing a particulate medium within a container, saidmedium adapted to support the surgical object and comprising anaggregation of substantially rigid particles of sufficiently small sizeto substantially conform to an outer surface of the surgical object, (b)placing an already formed surgical object within the container, thesurgical object being composed of a biocompatible thermoplasticmaterial; thereafter (c) compacting the particulate medium to rigidifythe medium to conform the particulate medium to the outer surface of thesurgical object to constrain the surgical object within the container;and then (d) applying heat for annealing the surgical object.
 15. Amethod as recited in claim 14, wherein said step of compacting thesupport medium into a substantially rigid mass comprises the step ofcreating a difference between the pressure exterior to the aggregationof particles of the particulate support medium and the pressure insidethe particulate support medium.
 16. A method as recited in claim 14,wherein said container is substantially rigid on all sides and saidparticulate support medium and said container have relatively differentcoefficients of thermal expansion changing the temperature of saidparticulate support medium and said container.
 17. A method as recitedin claim 14, wherein said surgical object is fully covered by saidparticulate support medium.
 18. A method as recited in claim 14, whereinthe step of placing the surgical object within the container furthercomprises the step of pressing the surgical object into the particulatesupport medium.
 19. A method of annealing a thermoplastic surgicalarticle which is subject to deformation upon annealing, said methodcomprising:a) supporting the surgical article by placing the article ina particulate support medium which comprises an aggregation ofsubstantially rigid particles of sufficiently small size tosubstantially conform to an outer surface of the surgical article, thesupport medium being within a container, and thereafter rigidifying thesupport medium by compacting means to conform the support medium to theouter surface of the surgical article; and then b) applying heat forannealing the surgical article; step (a) being performed so as toachieve a reduction of the deformation of the article as compared withan annealing method in which the article is supported by anon-particulate rigid holding means.
 20. The method of claim 19, whereinsaid reduction of the deformation of the article is a reduction of atleast about 35%.
 21. The method of claim 19, wherein said reduction ofthe deformation of the article is a reduction of at least about 46%. 22.The method of claim 19, wherein the average deformation is no more thanabout 5.0%.
 23. The method of claim 19, wherein the average deformationis no more than about 3.2%.
 24. The method of claim 19, wherein saidcompacting means comprises creating a difference between the pressureexterior to said aggregation of particles in the support medium and thepressure within said aggregation of particles of the support medium. 25.The method of claim 24, wherein said container is flexible.
 26. Themethod of claim 19, wherein said container is substantially rigid on allsides and said support medium and said container have relativelydifferent coefficients of thermal expansion, and said compactingcomprises changing the temperature of said support medium and saidcontainer.